The field of the invention is positron emission tomography (PET) scanners, and particularly circuits for detecting annihilation events by sensing the coincident receipt of gamma rays by a ring of detector crystals.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides most often employed in diagnostic imaging are fluorine-18 (.sup.18 F), carbon-11 (.sup.11 C), nitrogen-13 (.sup.13 N), and oxygen-15 (.sup.15 O). These are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid and glucose metabolism, and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners--each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilation events at each location within the scanner's field of view.
The PET scanner includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers are employed to reconstruct an image using well known computed tomography techniques.
To produce an image it is necessary to provide a number of gamma ray detectors in a ring which surrounds the patient. If events are detected within a small time window by two detectors located on opposite sides of the ring, then a coincident event is recorded. Coincidence detection circuits are connected to each of the detectors in the ring and they look for such coincident events in all of the possible detector pair combinations. For example, in prior systems such as that described by D. F. Newport et al. in "Coincidence Detection and Selection in Positron Emission Tomography Using VLSI", IEEE Transactions on Nuclear Science, Vol. 36, No. 1, February 1989 the detector ring includes sixteen separate detector modules, and fifty-six separate module pairs are continuously monitored for coincidence events by the coincidence detector circuit. The coincidence detector circuit, therefore, has one complete channel for each possible detector pair to be examined, and these examinations are performed in parallel once each 256 nanosecond sample period. Even if multiple coincidence events occur during a given sample period, only one is recorded by such prior detector circuits.
To increase image resolution and reduce scanner deadtime, the number of detector modules must be increased. In the preferred embodiment of the present invention, for example, fifty-six separate detector modules are provided in the ring. To detect coincidence events for such a system using prior techniques would require 700 separate channels. Such a coincidence detector circuit is too complex and costly even when VLSI circuit technology is employed.